The present invention relates to the production of a cellular or foamed polymeric material, and more particularly, the invention relates to a method of production of a cellular thermoplastic polymeric material such as an olefin polymer or copolymer, or a cellular elastomer or a thermoplastic rubber, which material, after the grafting on of a silane compound, may crosslink in the presence of moisture. The cellular material may, for example, be utilized to form a coating or sheath on a linearly extended substrate member, for instance an electric cable, lead or conductor, or a tube.
Processes are already known for the production of materials which crosslink in the presence of moisture. As a preparatory step, silane is first being grafted onto the molecules of a polymeric base or host material and subsequently the material is exposed to moisture to obtain crosslinking (see, for example, German printed patent applications Nos. 1,794,028 and 2,439,513, as well as U.S. Pat. Nos. 3,646,155 and 4,058,583). It is also known to expand material for purposes of obtaining foam prior to crosslinking in the presence of water, in that an expanding or blowing agent has been added to the raw material (see German printed patent application No. 23 10 040). The expanding or blowing agent is activated by heat, releasing a gas, e.g. nitrogen or carbon dioxide, which produces cells in the material. This known method is disadvantaged, however, in that the expanding agent, e.g. azodicarboxylamide (azodicarbonamide), dinitrosopentamethylenetetramine, sodium bicarbonate or ammonium carbonate, or a decomposition product thereof, will normally remain in the material and may, for example, produce an appreciable modification in its electrical properties. This means that for certain purposes the cellular material is rendered unsuitable.